The introduction of functionalities into non-polar chains (such as polyolefins) would make it possible to greatly modify the properties of the polymers in terms of hardness, adhesion, barrier properties and surface (coloration), but also in terms of rheology or of miscibility with the other polymers, while at the same time retaining the mechanical properties associated with polyolefins. Conversely, the introduction of non-polar olefin units into polar polymer chains (in particular (meth)acrylic polymers) would make it possible to improve their mechanical properties, their flexibility properties and their properties of resistance to chemical products. The synthesis of functional polyolefins is thus of great interest.
However, the efficiency of the copolymerization of polar and non-polar olefins is limited by the difference in reactivity of the comonomers: non-polar olefins are generally polymerized by catalysis, whereas polar monomers are polymerized by radical or ionic polymerization. In order to introduce functionalities onto polyolefins, two strategies (catalytic or radical) have therefore been envisioned.
Catalytic methods of polymerization and copolymerization of polar and non-polar olefins have been widely described. Some report the use of organometallic catalysts of metals belonging to group IV (Ti, Zr, etc.). Unfortunately, these highly oxophilic systems are rapidly poisoned by the functional group of the polar monomers. In order to remedy this poisoning, some have chosen to add a cocatalyst (of alkylaluminum type) to their system (Marques M. M. et al, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 37, 2457-2469, 1999; Aaltonen P. et al, Macromolecules 1996, 29, 5255-5260), with the aim of chemically protecting the polar functional group. These systems are then capable of copolymerizing ethylene and monomers of hydroxy- or carboxy-alpha-olefin type (for example: 10-undecen-1-ol). The copolymers obtained contain a maximum of 10 mol % of the polar monomer. In this case, the major drawback of the system is the need to add a cocatalyst in order to protect the polar functional group of the polar olefin, which renders the system obsolete since the cocatalyst should be used stoichiometrically with the polar monomer.
The same observation can be made with regard to certain nickel-based systems (Carlini C. et al, Macromol. Chem. Phys. 2002, 203, 1606-1613). The addition of methylaluminoxane (MAO) to the system as cocatalyst also acts as protection for the polar functional group. These systems then make it possible to copolymerize ethylene and methyl methacrylate (MMA) with degrees of MMA insertion ranging from 3 mol % to 80 mol %. However, the copolymers obtained exhibit either a very predominant incorporation of methyl methacrylate (between 61 mol % and 82 mol %) but with low molar masses (less than 30 000 g/mol) and a high polydispersity index (greater than 30) with an Ni(II) complex, or a very low incorporation of methyl methacrylate (between 3 mol % and 7 mol %) for copolymers of high molar masses (between 49 000 and 290 000 g/mol) with an Ni(0) complex.
Other systems, based on copper (U.S. Pat. No. 6,417,303, U.S. Pat. No. 6,479,425, Pracella M. et al, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 45, 1134-1142, 2007), also make it possible to synthesize ethylene/acrylate or ethylene/methacrylate copolymers, but require the use of a cocatalyst of alkylaluminum type (MAO).
Other teams have reported the use of organometallic catalysts of less oxophilic metals belonging to group X (Ni, Pd), without protection of the polar functional group by an alkylaluminum (Mecking S, Coordination Chemistry Reviews 2000, 203, 325-35; Johnson L. K et al, Chemical Reviews 2000, 100, 1169-1203; Boffa L. S. and Novak B. M., Chem. Rev. 2000, 100, 1479-1493). These nickel- and palladium-based systems (also described in the following documents: WO0192348, WO0192354, WO02059165, WO9623010, WO9842664, WO2004101634, U.S. Pat. No. 6,777,510) are limited to polar monomer incorporations of a maximum of 15 mol % since they result in copolymers of which the polyethylene part is rich in branching (approximately 100 branches per 1000 C) and of which the polar functional group is always inserted at the end of branches of the polymer. These systems can be used without a cocatalyst, but make it possible to copolymerize only a restricted number of polar monomers, such as functionalized norbornenes or acrylates.
Other palladium-based systems have been described (WO0192342, Liu S. et al, Organometallics 2007, 26, 210-216, Skupov K. M. et al, Macromol. Rapid Commun. 2007, 28, 2033-2038), which incorporate the polar monomer into the backbone of the polymer chain so as to give ethylene/alkyl acrylate copolymers containing up to 17 mol % of alkyl acrylate, in isolated units in the copolymer chain. The drawback of these systems is that they produce polymers of low molecular masses (less than 104 g/mol, or even than 103 g/mol as soon as a notable proportion of polar monomer (at least 10%) is integrated into the copolymer).
The use of these known catalytic systems does not make it possible to obtain copolymers having sequences in the form of polar olefin blocks and non-polar olefin blocks, with balanced proportions of each constituent within the copolymer, for molecular masses greater than 10 000 Da.
The second strategy used for the copolymerization of polar and non-polar olefins uses radical chemistry. It is mostly industrial processes which make it possible to obtain, for example, copolymers of ethylene and of vinyl acetate (ethylene vinyl acetate or EVA, vinyl acetate/ethylene or VAE copolymer). However, these processes do not make it possible to obtain a controlled microstructure of the polymers; in polymers obtained by radical polymerization, the comonomers are distributed randomly in the polymer chain, which has branches; the polymerization conditions are restrictive, in terms of temperature (which can go up to 350°) and of pressure (up to 3000 bar).
Other known radical systems make it possible to copolymerize polar and non-polar olefins under milder conditions. MMA/ethylene and MMA/1-hexene copolymers are obtained using the radical initiator AIBN in the presence of the comonomers (Nagel M. et al, Macromolecules 2005, 38, 7262-7265; Liu S. S. and Sen A. M., Journal of Polymer Science Part A: Polymer Chemistry, Vol. 42, 6175-6192 2004). MMA/1-octene and methyl acrylate (MA)/1-octene copolymers have been obtained in the presence of a copper system of “atom transfer radical polymerization” ATRP type (Venkatesh R. and Klumpermann B., Macromolecules 2004, 37, 1226-1233). MA/hexene and MA/norbornene copolymers have been obtained by radical polymerization using a palladium complex (Tian G. et al, Macromolecules 2001, 34, 7656-7663).
The major drawback of these systems comes from the fact that no sequence of non-polar olefins in the form of a block has been observed. Only isolated units of non-polar olefins in a polar olefin chain are observed in the copolymer.
It must be concluded that no known system makes it possible to suitably copolymerize non-polar and polar olefins. Catalysis makes it possible to obtain polyolefins containing a limited level of polar monomer, while radical polymerization makes it possible to obtain polar polymers containing a limited level of olefin.